1. Field of the Invention
The present invention relates to a zoom lens system suitable for a single lens reflex (SLR) camera, and especially suitable for a digital SLR camera.
2. Description of the Prior Art
A conventional zoom lens system for a SLR camera has a zoom ratio of approximately 2 to 3, and has an angle-of-view covering from a wide-angle range to a telephoto range (a standard zoom lens system). In such a zoom lens system, a retrofocus two-lens-group zoom lens system, including a lens group having a negative refractive power (hereinafter, a negative lens group) and a lens group having a positive refractive power (hereinafter, a positive lens group), in this order from the object, has often been employed.
On the other hand, a conventional zoom lens system of a digital SLR camera has widely employed an imaging device having an image plane which is smaller than that of a silver-halide-film SLR camera with a 35 mm-film format. This is because it has been difficult to increase the size of an imaging device in a digital SLR camera.
Accordingly, the angle-of-view of a digital SLR camera becomes narrower than that of a silver-halide-film SLR camera even in the case where a common exchangeable lens which can also be used with a silver-halide-film SLR camera is mounted thereon. Therefore a zoom lens system of a shorter focal length is preferable and suitable for a digital SLR camera.
Furthermore, in a digital SLR camera, if an attempt is made to design a lens mount and a flange back so that these lens mount and flange back can also be used for a widely-used exchangeable lens of a silver-halide-film SLR camera, a longer back focal distance is required, compared to the focal length of the exchangeable lens.
As explained, it is more difficult for a digital SLR camera to make the focal length at the short focal length extremity shorter than for a silver-halide-film SLR camera to do the same. Moreover, pictures taken by a digital SLR camera have frequently been enlarged and viewed on a personal computer and so forth, though the image plane of an imaging device is smaller than that of a silver-halide-film SLR camera with a 35 mm-film format. Due to these reasons, an imaging device of a digital SLR camera is required to have higher optical performance over the entire image plane of the imaging device.
However, in a conventional retrofocus zoom lens system of the two-lens-group arrangement having a negative lens group and a positive lens group, in this order from the object, the number of lens elements is larger; and miniaturization of the zoom lens system and the cost reduction on manufacturing the zoom lens system have not been sufficient. Moreover, the zoom ratio is less than 2, and the angle-of-view at the short focal length extremity is insufficient, i.e., the focal length cannot be made sufficiently shorter thereat.
The present invention is applied to a two-lens-group zoom lens system which includes a negative first lens group and a positive second lens group, in this order from the object, and has a smaller imaging device. In such a zoom lens system, the objective of the present invention is to provide a zoom lens system of a two-lens-group arrangement with the following features by suitably distributing refractive power over each lens group:
(i) the angle-of-view of 75° or more is attained;
(ii) the zoom ratio is 3 or more;
(iii) the focal length at the short focal length extremity is longer, compared to the back focal distance;
(iv) off-axis aberrations at the short focal length extremity and the on-axis aberrations at the long focal length extremity are maintained in a well balanced manner, while peripheral illumination at the short focal length extremity is secured; and
(v) the zoom lens system is miniaturized with lower production costs.